Patman is bringing up some very valid points about extended oil drains.
Many are changing oil at a drop of the hat, some trying to extend the drain and push the limits.
Each time you change brands of oils, you do several things.
Each type of oil reacts differently on how well it will fight acids and oxidation. Many do not hold up very well, therefore leaving residue and when putting a fresh oil, it immediatly has to start off by fighting off the acids left behind from the prior oil. This process, immediatly starts depleting additives on the new oil, therefore can shorten the extended drain capabilities. This also can show a whole different senerio on an oil analysis report as not holding up that well, when in fact it might.
So how do you change over and avoid this? Purge your engine prior to change over or do a short drain with the new oil and allow it to help purge the old oil and contaminents before trying to start extending drains.
In my case for example, I advise my customers to put in some of Schaeffers 131 Neutra for at least 500 miles prior to changeover so that schaeffers oil doesn't have to expend itself trying to neutralize acids and contaminents not handled from the previous one.(Auto-rx may also be an alternative for the neutra) Once the change over, now the oil can start out with a fairly level playing field. If a person opt's not to purge the engine, then we expect they may experience any number of minor problems, which will normally go away after the 2nd or 3rd oil change.
For extending drains, it is advisable to start out with trending your oil. First oil change with the new oil should follow your standard drain interval with oil analysis, next time, at normal drain interval, draw another sample, and see if good for continued use, also note to the lab that you are looking at extending drains and would like their suggestions on how far.
Continue this process until you either reach it's limits or reach your desired drain interval.